Myeloperoxidase, a heme enzyme of polymorphonuclear neutrophils, will be purified and examined by a variety of spectroscopic means. Special characteristics of myeloper-oxidase is its ability to catalyze the oxidation of halide ion, such as chloride, bromide, and iodide ions, to hypohalite by hydrogen peroxide to kill the ingested organism. Since the heme group is the active center of this enzyme, the electronic properties of the heme irom, the environment of the heme group, and the effect of substrate binding (particularly chloride ion) on these properties will be elucidated in order to obtain the vital informations about the mechanism of action of the enzyme. The primary goals of this proposal are: (1) determination of the chloride binding site, (2) characterization of the structure of the heme group, (3) determination of the mode of the interactions between the heme group and apo-protein moeity, and (4) evaluation of the inequivalence between the heme groups by systematic utilization of light absorption, electron paramagentic resonance, proton- and chlorine-nuclear magnetic resonance, resonance Raman scattering, and kinetic spectroscopies. Elucidation of the molecular mechanism of the enzyme action of myeloperoxidase is essential for our understanding the bacteriocidal function of polymophonuclear leukocytes, which is vital for survival of mammalian species against bacterial infections.